Electromagnetic radiation is emitted when an atom or molecule comes down from an upper to a lower energy state in accordance with the laws of quantum electrodynamics, conserving momentum and energy. In a laser, the best-known case of coherent electromagnetic radiation generated by atomic systems, this is made to occur as a resonance process via stimulation of excited atoms by incident photons having energy the same as that of the photons that result from the de-excitation. The demands of pumping, population inversion, stimulation, and multiplication of photon intensities without excessive losses, essential to the generation of coherent radiation can be met with relative straight-forwardness for radiation in the infrared and visible regions of optical frequencies. Various techniques were applied successfully in this effort that led to the development of lasers of a large range of wavelength. Special efforts further led to a variety of lasers and to lasers capable of very high power output. Several modes of soft X-ray lasers were built over the past years, with ionized vapors in the plasma state employed as the laser media. M. D. Rosen et al, Physical Review Letters, Volume 54, (1985), pages 106-109, describe an exploding foil technique of achieving such a soft X-ray laser. Despite such successes, generation of X-ray lasers has remained restricted to relatively low photon frequencies.
Production of lasers of increasingly higher frequencies is beset with rising difficulties. For example, A. V. Vinogradov and I. I. Sobel'man, Soviet Physics JETP, Volume 36, (1973), pages 1115-1119, give a presentation of the problems visualised of creating laser radiation sources in the far ultraviolet and X-ray regions. Energy-input demands are higher at increased radiation frequencies. Population inversion cannot be maintained for long enough periods because the rate of spontaneous emission relative to stimulated de-excitation increases as the third power of frequency of the emitted radiation. Only excited states of ultra short life times have large enough energy widths that provide adequate resonance cross sections for stimulated emission. Conventional reflection techniques do not apply for X-ray wavelengths. Consequent of these difficulties, the lasers built hitherto are limited to wavelengths over a few nm, which correspond to photon energies below about 1 keV.
In view of the great significance of X-ray lasers in science, technology, and medicine, persistent efforts continue to be undertaken for developing lasers operating at higher photon energies. In illustrating this, D. L. Mathews and M. D. Rosen, Scientific American, December 1988, pages 86-91, review various efforts undertaken in laboratories worldwide for producing X-ray lasers of short wavelengths. Also, W. T. Silfvast, Selected Papers on Fundamentals of Lasers, SPIE Milestone Series, Vol. MS 70 (SPIE Optical Engineering Press, 1993) presents a more recent account of such ongoing efforts.